This invention relates to a new type of activated carbon having a high surface area necessary for many filtering applications but having a compressive strength in structured form which is exceptionally high.
An enormous amount of literature has developed over the years relating to a wide variety of types of activated carbon. The latter finds uses in a wide variety of applications, e.g., filtering (gases, particulates, liquid droplets), purifying, exchanging, etc., in both gaseous and liquid media. See, e.g., U.S. Pat. Nos. 4,397,907, 4,362,646, 3,322,489, 3,769,144, 4,342,574, 4,444,574, 3,871,841, 4,167,482, 3,986,835, 4,350,672, 3,671,385, and 4,285,831. In most of these applications, the activated carbon has a high surface area, e.g., 300-2,000 m.sup.2 /g in order to maximize adsorption/absorption effects. Many methods are known for achieving increased surface area; see, e.g., U.S. Pat. Nos. 4,362,646 (especially columns 7 and 8) and 4,285,831 (especially columns 5-7). In the vast majority of cases, the activated carbon material has very low compressive strength, e.g., is in the form of a woven or non-woven fabric, a felt, powder, granulate, often sandwiched between porous substrates, etc. This poses significant problems in many environments, e.g., those wherein certain pressure drops can occur (explosions, violent weather conditions (tornados, etc.). Furthermore, this characteristic of activated carbon presents problems in packaging, replacing spent material, etc.
The molecular adsorption capacity of these conventional filter systems is determined primarily by the total surface area of the carbon per unit volume. In order to maximize this surface area, particle size, packing density and configuration are conventionally varied. However, due to the inherent nature of powder packing, an increase in surface area usually results in an increased air flow resistance. Consequently, there is a natural limitation on the level to which surface areas can be increased while maintaining sufficiently low air flow resistance.
On the other hand, carbon materials of exceptionally high compressive strength are also known for particulate filtering applications, e.g., nuclear contamination or chemically toxic molecules. These are the carbon bonded carbon fiber composites, ceramic bonded ceramic fiber composites or carbon bonded ceramic fiber composites. In such materials, the typical aim is for high compressive strength in combination with high porosity, low air flow resistance, low outgassing, and good thermal insulating properties. However, since particulate filtering is the major application of such composites, the low surface areas involved (typically less than or about 1 m.sup.2 /g specific surface area) are acceptable. For a thorough discussion of the preparation and characteristics of such composites, see, e.g., U.S. Pat. Nos. 4,500,328, 4,391,873 and 4,152,482 and "Low Density Carbon Fiber Composites," Reynolds et al, Union Carbide Nuclear Division, Informal Report No. Y/DA-6925, October 1976, whose disclosures are entirely incorporated by reference herein.
Heretofore, it has never been suggested that these carbon-containing composite materials could be exposed to carbon "activation" conditions for increasing surface area and even more certainly there has never been a suggestion that such materials might be successfully exposed to activation treatments while retaining their compressive strength and low air flow resistance. It would be expected that the loss of the significant amount of carbonaceous material needed to achieve the necessary surface area increase to permit the carbon composite to achieve the characteristics necessary in an activated carbon, would have a significant deleterious effect on the structural integrity of the composite thereby significantly lowering its compressive strength and/or increasing air flow resistance due to such structural changes.
For example, in U.S. Pat. No. 4,350,672, column 1, there is reported the development of stresses and/or cracks in binders and possible weakenings of bonds between fibers in the matrix upon attempted modifications of the basic processing conditions and structures in carbon-carbon composites. Note especially lines 37-41. These prior art difficulties contribute to the fact that heretofore such composites have not been utilized in a wide variety of applications.